1. Field of the Invention
The invention relates to the obtaining of digital recording data for rastered color separations in the field of printing techniques.
2. Description of the Prior Art
In the field of printing techniques, it is known to prepare color separations in a rastered manner for the purpose of multi-color-printing, whereby the raster can differ from color separation to color separation. In the case of the four-color-printing process, for example, the rastered color separations can be obtained from the scanning signals, with separate digital intermediary storage of the individual raster-spot-configurations and with a relatively complex recording unit, such as it is described in the British Pat. No. 1,355,540. In the case of this process, the color separations are individually recorded one after the other, whereby a separate scanning process is required for each color separation.
However, in modern printing techniques, which more than before make use of digital image processing, for example British Pat. No. 1,407,487 or British Pat. No. 1,404,672, it is desirable to obtain the color separations as quickly as possible, in order to retrieve them from digital memories for further processing such as, for example, for the computer controlled preparation of printed pages, for engraving processes or for electronic photo-compositions. The data of the color separations are present in the memory unit in digital form and, according to the distance of the scanning points on a scanning line and the distance of the scanning lines from one another, furnish an orthogonal network of scanning points. This network is the same for all color separations and therefore completely unsuitable for the multi-color printing-process. If all the color separations were to be prepared with this raster then, on superimposing the color prints, an image with an intolerable interplay of colors would result since alignment- and recording-errors are unavoidable due to the distortion of the paper and the mechanical tolerances in the printing plants. The interplay of colors takes place for the following reason namely, since based on these errors, the color-points are printed once on top of one another and once next to one another.
From the German Patent Application Open to Inspection No. 23 11 678, we already known of a device for preparing color separations of a multicolored image, in the case of which the color separation signals, obtained through optical-electrical scanning of an original image, are digitally fed in parallel into a memory unit, in order to be read sequentially after having been recorded in the memory unit and in order to be then sequentially recorded on the periphery of a recording drum. Similar processes are disclosed in the U.S. Pat. No. 3,878,559 and in the British Pat. No. 1,382,124. The disadvantage of these processes consists therein in that the recorded color separations are not in raster form, since a D/A-transducer is switched into circuit beyond the memory-unit. In the known process, the scanning raster is therefore again eliminated and the color separations are recorded in analog manner, that is to say not in a raster form. As far as the printing process is concerned, a separate process follows thereupon, namely in that from the color separations which are not present in raster form, rastered color separations are obtained through a re-copying process by means of side-by-side rotated contact rasters.
Furthermore, according to the German Pat. No. 1,112,407, a raster process is known which is used particularly in integlio-printing processes in which two color separations each (magenta and yellow or cyan [-blue] and black or two others in each case) are prepared with two similar rasters. These rasters are characterized in that the one raster to be used for the two first mentioned color separations can be expanded in horizontal direction, that is to say in transversal direction to the lines and is held taut in vertical direction, that is to say in longitudinal direction of the lines, whereby the raster distance in longitudinal and transverse direction show in each case the behaviour of whole numbers. In conforming manner, the raster for the two other color separations is obtained in a similar manner, namely in that the raster is expanded here in the opposite sense, that is to say in the longitudinal direction of the lines and in that the raster is held taut in the same manner as before. In a practical case of application, for exemple, in the case of the "Helio-Klischograph" engraving machine of the applicant, the recording of the raster spots take place thereby in that an engraving needle oscillates with a fundamental frequency which furnishes a desired time derived and therefore localized distance of the raster spots, whereby the continuously furnished image signal is superimposed over the fundamental oscillation. This process is carried out once for each color separation, that is to say all together four times.
This has the disadvantage that the obtaining of the complete four color-separation-signals is possible only in form of the printed form and that this is time-consuming owing to the fact that scanning operations are repeatedly required.